DBC frontend

The DBC (CAN database) frontend (taktora-idl-dbc (BB_0119)): parses .dbc text into a typed model and lowers it onto an idl-core Module plus a physical-layout sidecar. DBC is the natural first frontend because it is bounded by construction — every message has a fixed DLC and every signal a fixed bit width.

Feature: DBC frontend FEAT_0113
status: implemented
satisfies: FEAT_0110
is satisfied by: REQ_0952, REQ_0953
is refined by: ADR_0123
is implemented by: BB_0119

A new crate taktora-idl-dbc that parses .dbc databases and lowers them onto the message-plane IR. It proves the description → IR → codegen pipeline against a frontend whose boundedness invariant is never even at risk.
Requirement: DBC parse to a typed model REQ_0952
status: implemented
satisfies: FEAT_0113
is verified by: TEST_0929
links outgoing: BB_0119, TEST_0929

taktora-idl-dbc shall expose parse(text) -> Result<DbcDatabase, ParseError> producing a faithful in-memory model of the database: messages (with CAN id, extended-id flag, DLC, and sender), signals (start bit, bit length, byte order, signedness, factor, offset, min, max, unit), value tables, and signal multiplexer roles (multiplexor / multiplexed / none).
Requirement: DBC lower to IR plus layout sidecar REQ_0953
status: implemented
satisfies: FEAT_0113
is verified by: TEST_0935, TEST_0929
links outgoing: BB_0119, TEST_0929, TEST_0935

taktora-idl-dbc shall expose lower(&db, module_name) -> Result<LoweredDbc, LowerError> producing two outputs: an idl_core::Module (each message a struct, each signal a field, each value table an enum) and a DbcLayout sidecar carrying the physical placement the IR deliberately excludes (start bit, bit length, byte order, factor, offset). The lowered Module shall always pass Structural validation befor... (REQ_0947). The sidecar shall be keyed by the same source names the Module carries so a backend can rejoin the logical and physical halves. Multiplexed signals shall lower into plain fields of one struct this round.